1. Field of the Invention
The present invention relates to a method of controlling the defects of a silicon single crystal, and in particular to a method of controlling the defects of a silicon single crystal, having a superior inhibiting effect on the defects introduced during the growth of the silicon single crystals, i.e., the so-called grown-in defects.
2. Description of the Related Art
As the grown-in defects generated during the growth of a CZ-silicon crystal degrade the reliability of the gate oxide film of a MOS device, it is necessary to reduce the grown-in defects. It is known that the densities of the grown-in defects are dependent on the heat history of a growing crystal, and thus the improvement on the optimization of the temperature distribution and the pulling speed of a single crystal is carried out by trial and error. For example, the reliability of the oxide film of a MOS device is strongly affected by the defects introduced during the growth of a silicon single crystal, which are detected as the LSTD (Laser Scattering Temograph Defect). Accordingly, the control and reduction of the formation of the defects are big problems to be solved. It is known by experiences that the control of the cooling process after crystallization has effect on the formation of the defects. A technique of inhibiting the defects by managing the temperature and the pulling speed is disclosed in the Unexamined Japanese Patent Publication (kokai) Nos. Hei. 5-70283, Hei. 5-56588 and Hei. 6-279188.
For the temperature distribution in a single crystal, the temperature at the crystallization end in contact with the silicon melt is highest, and is gradually reduced when departing from the crystallization end. Accordingly, it is thought that the control of cooling speed can be realized by a furnace structure that can properly set the axial temperature distribution and by properly setting the pulling speed of single crystals. That is, the change of the temperature in the axial direction is generally not a straight line, and the cooling speed passing through each temperature zone of a single crystal changes in a complicated manner. Moreover, as the heat history of each temperature zone influences the phenomena generated in the next temperature zone, the setting of conditions that renders a single crystal an appropriate temperature distribution is performed by trial and error which needs considerable labors. By means of the method disclosed in the prior art, it is not possible to predict the defect-forming density caused by the temperature history in advance, and thus considerable man-hour is necessary, and thus in fact an expected result can not be obtained.